The present invention relates generally to isolation apparatus for multiple antenna installations and more particularly for certain installations such as portable aircraft control facilities having many closely spaced antennas and other auxiliary apparatus such as a wind sensor together with a plurality of respective radio transceivers. This equipment is included in a self-contained unit which as a consequence of relatively close spacing normally has undesired interaction between the antennas, the vertically disposed feed lines, and the wind sensor control cable.
It is well-known that the electrical characteristics of an antenna can be adversely affected when situated in the vicinity of other antennas, metal masts, metal surfaces, electrical wiring or transmission lines. For example, the antenna impedance and radiation characteristic may be substantially changed due to the parasitic excitation and reradiation by and from the other conductors.
In certain military installations, such as in aircraft control towers where many closely spaced antennas for different frequencies are employed, acceptable antenna performance is difficult to obtain. The various antennas interact with one another, resulting in modified impedances and radiation characteristics.
When attempting to analyze the radiation characteristics of a complicated system such as a portable aircraft traffic control facility for military use, and more particularly a facility such as the AN/TSQ-97, not only the antennas, but the entire structure consisting of antennas, cables, wind sensor, console ground and metal masts, have to be regarded as a complex system for radiating and absorbing electromagnetic waves. Accordingly, it is very difficult if not impossible to predict theoretically the radiation characteristics of such a complex structure. It is practical, however, to measure the antenna patterns in the horizontal and vertical planes, but the experimental determination of radiating characteristics is a time consuming procedure when the operating bandwidth is great and therefore such a determination may have to be based upon measurements made at only a few selected frequencies.
One approach to the problem of improving the radiation properties of a multi-antenna system is to locate all of the antennas on a common vertical axis. Such an arrangement provides a substantially omnidirectional pattern in the horizontal plane. Radiation in the vertical plane, however, depends upon current distribution, antenna height above ground and operating frequency. Over all, such an antenna system provides relatively better performance in comparison to any radiating system having antennas mounted in a broadside relationship.
Where, however, a broadside array is desirable, notwithstanding the advantages gained by a vertical in-line array, it is possible to reduce the strength of the induced feed line currents by careful arrangement of radiators and avoiding resonant lengths of feed lines; however, in a case of systems operating over a broad frequency range, e.g., over two octaves for example, it is very difficult to pick optimum lengths of cables, etc.
One means which is helpful in optimizing broadband radiating systems is to insert separate high impedance broadband cable chokes in series with the antenna feed lines. When connected in series with feed lines, the high impedance property of the cable choke can tend to suppress feed line current flowing on the outside of the feed line induced by the impressed electrical field. Illustrative examples of such apparatus is taught for example, in U.S. Pat. No. 3,879,735 of Donn V. Campbell and James J. Arnold, entitled "Broadband Antenna System With Isolated Independent Radiators;" and U.S. Pat. No. 3,961,331 of Donn V. Campbell, entitled "Lossy Cable Choke Broadband Isolation Means for Independent Antennas," both assigned to the assignee of the present invention. The cable chokes illustrated therein consist of a high inductance made by winding a plurality of coaxial cables in the shape of a helix and with several separate cable chokes being provided in those cases in which different frequency ranges are involved. For example, one choke may impede feed line current at frequencies between 30 and 80 MHz while another choke may be designed for the frequency range of 200-400 MHz. At VHF frequencies, the choke would normally be wound on a magnetic core such as ferrite in order to maximize the inductance of the cable choke while at UHF frequencies, the magnetic core is usually deleted.